A common problem in the preparation of fine particles is their tendency to agglomerate, this is generally believed to be as a result of their high surface area to volume ratio which causes the particles to have a relatively high surface energy. It has been speculated that the agglomeration could be as a result of electrostatic forces between the particles, cold sintering, or van der Waals interactions. The result of this agglomeration is that fine particles are generally found as clusters, platelets or strings of the fine particles, and not as isolated fine particulate matter.
It would be advantageous to provide fine particles which are not agglomerated as agglomeration reduces the surface area of a given mass of fine particles. This would provide systems where either less fine particulate matter would be required for use in surface chemistry applications, or the same mass of fine particles could be used, but faster reaction rates observed as a result of the larger surface area available for reaction or physical absorption. The above properties are of particular importance where the fine particles are being used as catalysts as it is the surface chemistry of the particles, and hence their surface energy, which promotes the reactions.
Further, un-agglomerated fine particles will disperse more efficiently in liquids, the resulting colloids having the properties of a “solution” of the fine particle, as though the particles were not merely suspended in the liquid, but dissolved therein. The ability to form such colloids enhances the ease and speed of reactions using the fine particles; as the particles can be intimately mixed with the reactants in solution. With agglomerated fine particles it is generally necessary to sonicate a suspension of the agglomerated fine particles, and often to add a dispersant, in order to produce a stable colloid. This places limitations upon the presentation of the fine particles to any medium with which they are to interact. As a result, the formation of liquid suspensions in this way can result in fine particles with altered surfaces, and reduced surface energy, making them less suitable for their intended application. As noted above, dispersants such as surfactants can be used to stabilise the suspensions, or to help the fine particles form coatings. The use of dispersants is undesirable as not only does their use increase the cost of the product, but the dispersant also acts to reduce the surface energy of the fine particles, retarding, reducing or degrading their utility in surface chemistry applications.
Where the particles are sintered, even sonication will be unlikely to separate the fine particles. The energy provided by ultrasound would be insufficient to break the bonds. Without being bound by theory, it is thought that sintering occurs just after fine particle formation, as a result of the high temperatures at which the particles are formed in plasma processes. Further sintering may be observed as the particles cool, even at or near ambient temperature due to the high surface energy of the fine particles before they are contaminated (for instance with air). It has been postulated that fine particles formed from a gaseous or plasma state may even form bonds or links as the particles are produced.
Another advantage of producing unagglomerated fine particles would be their ability to pass through fine membranes and filters in the same manner as many large molecules. For instance, it could be of use to produce a colloid of fine particles in which the fine particles can pass across a permeable membrane under osmotic pressure.
It would therefore be desirable to provide fine particles which do not significantly agglomerate or sinter and which are stable in their pure form, with substantially all of their surface energy intact. The fine particles of the invention and the process and apparatus for preparing these are intended to ameliorate one or more of the above problems.